Method for decreasing surface defects of patterned resist layer

ABSTRACT

Disclosed is a method for decreasing the surface defects of a patterned photoresist layer on a substrate surface obtained by the procedure comprising the steps of (a) forming a photoresist layer of a positive-working chemical-amplification photoresist composition on the substrate surface, (b) patternwise exposing the photoresist layer to actinic rays, (c) subjecting the patternwise-exposed photoresist layer to a post-exposure baking treatment and (d) a development treatment. The improvement can be accomplished by bringing the photoresist layer after the post-exposure baking treatment into contact with an aqueous acidic solution having a pH of 3.5 or lower for 1 to 90 seconds. The acid contained in the aqueous acidic solution is preferably an aromatic sulfonic acid or, more preferably, a diphenyl ether sulfonic acid such as dodecyl(diphenyl ether) disulfonic acids.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a treatment method of aphotoresist layer formed on a substrate surface by using apositive-working chemical-amplification photoresist composition fordecreasing the surface defects in the patterned resist layer which canbe detected by using a special inspection instrument.

[0002] Along with the trend in recent years in the field ofsemiconductor devices toward a higher and higher degree of integration,mass production of LSIs of a design rule 0.18 μm has already beenindustrialized and mass production of LSIs of the design rule 0.15 μmwill shortly be on the start line.

[0003] While the process of photolithographic patterning of aphotoresist layer on the substrate surface is undertaken in themanufacture of semiconductor devices, it is essential that the light forthe patterning light exposure of the photoresist layer has a shortwavelength in order to accomplish a high pattern resolution of thepatterned resist layer. In this regard, the g-line light of 436 nmwavelength used in the early development stage of the photolithographictechnology as the patterning exposure light was replaced with the i-linelight of 365 nm wavelength which in turn has been replaced with the KrFexcimer laser beams of 248 nm wavelength constituting the major currentof the patterning exposure light in the modern photolithographictechnology. Furthermore, ArF excimer laser beams of 193 nm wavelengthare now expected to be the patterning exposure light of the cominggeneration and active development works are now under way for thephotolithographic processes utilizing not only the KrF excimer laserbeams but also the ArF excimer laser beams and photoresist materialssuitable for use in the process utilizing these excimer laser beams.

[0004] As is well known, namely, the photoresist composition used as themajor current in the early stage was the positive-working photoresistcomposition comprising a novolak resin as the resinous ingredient and anaphthoquinone diazidosulfonate ester compound as the photosensitiveingredient. Photoresist compositions of this type, however, could hardlycomply with the technological requirements in the photolithographicpatterning works using the KrF excimer laser beams or other lightsources of further shorter wavelengths. This problem of the abovementioned photoresist compositions can be overcome by the use ofso-called chemical-amplification photoresist compositions including thepositive-working chemical-amplification ones, in which thealkali-solubility of the resinous ingredient in the light-exposed areasis increased by reacting with the acid generated from aradiation-sensitive acid-generating agent, and the negative-workingchemical-amplification ones, in which the alkali-solubility of theresinous ingredient in the light-exposed areas is decreased by reactingwith the acid generated from a radiation-sensitive acid-generatingagent.

[0005] The requirements to be satisfied by the chemical-amplificationphotoresist compositions heretofore developed include highphotosensitivity to the patterning exposure light, high patternresolution and heat resistance of the patterned resist layer, focusingdepth latitude, orthogonality of the cross sectional profile of thepatterned resist layer and holding stability, i.e. the stability againstdegradation of the cross sectional profile of a patterned resist layerdue to contamination with an amine compound and the like during standingbetween patternwise light exposure and post-exposure baking treatment ofthe resist layer as well as the substrate dependency which meansadaptability of the photoresist composition to substrates having surfacelayers of different natures such as insulating materials, e.g., siliconnitride, semiconductor materials, e.g., polycrystalline silicon, andceramic materials, e.g., titanium nitride. Namely, the cross sectionalprofile of the patterned resist layer is affected by the nature of thesubstrate surface on which the resist layer is formed.

[0006] In addition to the above mentioned various requirements, a recenttechnological issue relative to the performance of achemical-amplification photoresist composition is the problem of surfacedefects which must be overcome in order to accomplish a high-qualitypatterned resist layer.

[0007] The above mentioned surface defect of a patterned resist layer isa disordered resist pattern such as infidelity of the resist pattern tothe photomask pattern, deposition of scums or dust particles,short-circuiting between resist pattern lines and so on and isdetectable by examining the patterned resist layer vertically from abovewith a special surface-defect detector (for example, Model KLA,manufactured by KLA Co.).

[0008] The yield of semiconductor devices is greatly decreased when thenumber of the above-mentioned surface defects is large and thesemiconductor device cannot exhibit excellent performance even when theconventional requirements for the photoresist composition are satisfied.Thus, one of the important technological problems to be solved urgentlyin the manufacturing process of semiconductor devices is how to decreasethe surface defects. Unless the problems of surface defects are solved,great difficulties would be encountered in the mass production ofsemiconductor devices.

[0009] The inventors previously could arrive at a success to reduceoccurrence of such surface defects of patterned photoresist layers tosome extent by modifying the formulation of the positive-workingphotoresist solutions. It is, however, a very desirable way in thephotolithographic patterning technology if the problems of the surfacedefects could be solved effectively without such a modification of theformulation of the positive-working photoresist solutions.

SUMMARY OF THE INVENTION

[0010] The present invention accordingly has an object to provide asimple and convenient method for the surface treatment of a photoresistlayer on the surface of a substrate formed by using a positive-workingchemical-amplification photoresist composition in order to efficientlyand reliably decrease the surface defects of the patterned resist layerwithout any modification of the formulation of the photoresistcomposition and the photolithographic processing conditions by using thephotoresist composition.

[0011] Thus, the method of the present invention for decreasing thesurface defects in a patterned resist layer comprises, in thephotolithographic patterning process of a positive-workingchemical-amplification photoresist layer on a substrate surfacecomprising the steps of forming a photoresist layer on the substratesurface, patternwise exposing the photoresist layer to actinic raysthrough a pattern-bearing photomask, subjecting the thus patternwiseexposed photoresist layer to a post-exposure baking treatment anddeveloping the photoresist layer after the post-exposure bakingtreatment with an aqueous alkaline developer solution, subjecting thephotoresist layer after the post-exposure baking treatment to an acidtreatment by bringing the photoresist layer into contact with an aqueoussolution of an acid.

[0012] The above mentioned acid treatment of the photoresist layer isconducted by dipping the substrate bearing the photoresist layer in anaqueous acid solution or spray-coating or flow-coating of thephotoresist layer with the aqueous acid solution. In this acidtreatment, the photoresist layer is kept in contact with the aqueousacid solution preferably for a length of time in the range from 1 to 90seconds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] As is described above, the scope of the inventive method fordecreasing surface defects of a patterned resist layer formed on asubstrate surface consists in the additional step of an acid treatmentof the photoresist layer after the post-exposure baking treatment butbefore the development treatment. Excepting for this additional step,the other steps of the photolithographic patterning procedure are justthe same as in the conventional procedure utilizing a positive-workingchemical-amplification photoresist composition.

[0014] The substrate material on which the photoresist layer is formedis typically a semiconductor silicon wafer. The photoresist compositionto which the inventive method is applicable is a positive-workingchemical-amplification photoresist composition comprising, as theessential ingredients, (A) a resinous compound substituted byacid-dissociable solubility-reducing groups for the hydroxyl groups ofthe resin and (B) a radiation-sensitive acid-generating agent. Besidesthe above mentioned two-component photoresist compositions, the methodof the present invention is applicable also to a three-componentpositive-working chemical-amplification photoresist compositioncomprising an alkali-soluble resinous ingredient such as novolaks andpolyhydroxystyrene resins, a low molecular-weight phenolic compound suchas bisphenols and trisphenols substituted for the hydroxyl groups bytert-butyloxycarbonylmethyl groups as the acid-dissociablesolubility-reducing groups, which can be dissociated by reacting with anacid to form free carboxyl groups resulting in an increase in thealkali-solubility of the compound, and a radiation-sensitiveacid-generating agent.

[0015] The component (A) above mentioned is a compound having phenolichydroxyl groups or carboxyl groups, of which the hydroxyl groups aresubstituted by acid-dissociable substituent groups capable of reducingthe solubility of the resinous compound in an aqueous alkaline developersolution and capable of being dissociated by reacting with the acidgenerated in the patternwise exposure to actinic rays to increase thealkali-solubility of the compound. The component (A) is usually apolymeric resinous compound but can also be a low molecular-weightcompound according to need.

[0016] The resinous compound as the component (A) is a homopolymer orcopolymer such as a polyhydroxystyrene of which at least a part of thehydrogen atoms of the hydroxyl groups are substituted byacid-dissociable solubility-reducing groups exemplified bytert-butoxy-carbonyl groups, tert-butyl groups, alkoxyalkyl groups suchas ethoxyethyl and methoxypropyl groups and cyclic ether groups such astetrahydropyranyl and tetrahydrofuranyl groups.

[0017] A particularly preferable resinous compound as the component (A)is a polyhydroxystyrene resin of which a part of the hydrogen atoms ofthe hydroxyl groups are substituted by substituent groups selected fromthe group consisting of tert-butoxycarbonyl, tert-butyl, alkoxyalkyl andcyclic ether groups. Particular examples of such apolyhydroxystyrene-based resinous compound include polyhydroxystyrenesof which 5 to 60% of the hydroxyl-hydrogen atoms are substituted bytert-butoxycarbonyl groups, polyhydroxystyrenes of which 5 to 60% of thehydroxyl-hydrogen atoms are substituted by tetrahydropyranyl groups,polyhydroxystyrenes of which 5 to 60% of the hydroxyl-hydrogen atoms aresubstituted by 1-ethoxyethyl groups, copolymers consisting of 10 to 49%by moles of hydroxystyrene units of which the hydroxyl-hydrogen atomsare substituted by tert-butoxy-carbonyl groups, 10 to 49% by moles ofhydroxystyrene units of which the hydroxyl-hydrogen atoms aresubstituted by 1-ethoxyethyl groups and 2 to 80% by moles ofunsubstituted hydroxystyrene units, copolymers consisting of 10 to 49%by moles of hydroxystyrene units of which the hydroxyl-hydrogen atomsare substituted by tert-butyl groups, 10 to 49% by moles ofhydroxystyrene units of which the hydroxyl-hydrogen atoms aresubstituted by, 1-ethoxyethyl groups and 2 to 80% by moles ofunsubstituted hydroxystyrene units, copolymers consisting of 10 to 49%by moles of hydroxystyrene units of which the hydroxyl-hydrogen atomsare substituted by tetrahydropyranyl groups, 10 to 49% by moles ofhydroxystyrene units of which the hydroxyl-hydrogen atoms aresubstituted by 1-ethoxyethyl groups and 2 to 80% by moles ofunsubstituted hydroxystyrene units and the like.

[0018] When the photoresist composition is to be used in thephotolithographic patterning works with ArF excimer laser beams as thepatternwise exposure light source, particularly preferable resinouscompounds as the component (A) in the photoresist composition includethose copolymers comprising the monomeric units of an acrylic acid esterof a polycyclic aliphatic hydrocarbon group and the monomeric units ofan acrylate ester as the constituents of the main chain structure, suchas copolymers of γ-butyrolacton-3-yl methacrylate and 2-methyladamantylmethacrylate, and resins having polycyclic aliphatic hydrocarbon groupscontaining acid-dissociable solubility-reducing groups such as thegroups derived from tert-butyl ester of norbornene-5-carboxylic acid asthe constituents of the main chain structure.

[0019] These resinous compounds can be used either singly or as acombination of two kinds or more according to need. It is preferablethat the weight-average molecular weight of the resinous ingredient asthe component (A) is in the range from 2000 to 50000 or, morepreferably, from 5000 to 15000 assuming that the photoresist compositionis of the two-component type although the applicability of the inventivemethod includes photoresist compositions of the three-component type asis mentioned before.

[0020] The component (B) as the other essential ingredient of thepositive-working chemical-amplification photoresist composition used incombination with the component (A) is a radiation-sensitiveacid-generating agent which releases an acid when irradiated withactinic rays. Various compounds known as an acid-generating agent in thechemical-amplification photoresist compositions of the prior art can beused here without particular limitations including diazomethanecompounds, nitrobenzyl compounds, sulfonate ester compounds, onium saltcompounds, benzoin tosylate compounds, halogen-containing triazinecompounds, cyano group-containing oximesulfonate compounds and the likeand particularly preferable among them are diazomethane compounds andonium salt compounds having a halogenoalkylsulfonic acid as the anion,of which the halogenoalkyl group has 1 to 15 carbon atoms.

[0021] Particular examples of the diazomethane compounds suitable foruse here include bis(p-toluenesulfonyl) diazomethane,bis(1,1-dimethylethylsulfonyl) diazomethane, bis(cyclohexylsulfonyl)diazomethane and bis(2,4-dimethyl-phenylsulfonyl) diazomethane.Particular examples of the above mentioned onium salt compounds includediphenyliodonium trifluoromethane sulfonate, (4-methoxyphenyl)iodoniumtrifluoromethane sulfonate, bis(4-tert-butylphenyl)iodoniumnonafluorobutane sulfonate, (4-methoxyphenyl)-diphenylsulfoniumtrifluoromethane sulfonate, (4-tert-butylphenyl)diphenylsulfoniumtrifluoromethane sulfonate and triphenylsulfonium nonafluorobutanesulfonate.

[0022] In the chemical-amplification photoresist composition to whichthe method of the present invention is applied, theseradiation-sensitive acid-generating agents can be used either singly oras a combination of two kinds or more. The amount of the acid-generatingagent as the component (B) in the photoresist composition is in therange from 0.5 to 30 parts by weight or, preferably, from 1 to 10 partsby weight per 100 parts by weight of the component (A). When the amountof the component (B) is too small, complete pattern formation can hardlybe accomplished while, when the amount thereof is too large, thephotoresist composition can hardly be obtained in the form of a uniformsolution due to the limited solubility of the compound to cause adecrease in the storage stability of the photoresist solution, if itever be obtained.

[0023] It is optional that the positive-working chemical-amplificationphotoresist composition to which the inventive method is applied isadmixed with various known additives such as carboxylic acids,phosphorus-containing oxoacids, amine compounds and the like each in alimited amount. It is of course convenient that the photoresistcomposition is used in the form of a uniform solution prepared bydissolving the above-described essential and optional ingredients in anorganic solvent. Examples of the organic solvents usable here includeketones such as acetone, methyl ethyl ketone, cyclohexanone, methylisoamyl ketone and 2-heptanone, polyhydric alcohols and derivativesthereof such as ethyleneglycol, ethyleneglycol monoacetate,diethyleneglycol, diethyleneglycol monoacetate, propyleneglycol,propyleneglycol monoacetate, dipropyleneglycol and dipropyleneglycolmonoacetate as well as monomethyl, monoethyl, monopropyl, monobutyl andmonophenyl ethers thereof, cyclic ethers such as dioxane and esters suchas methyl lactate, ethyl lactate, methyl acetate, ethyl acetate, butylacetate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate andethyl ethoxypropionate. These organic solvents can be used either singlyor as a mixture of two kinds or more.

[0024] In applying the inventive method to a photoresist layer of apositive-working chemical-amplification photoresist composition, aphotoresist layer formed on the surface of a substrate by using thephotoresist solution is patternwise exposed to actinic rays such as KrFexcimer laser beams through a pattern-bearing photomask followed by apost-exposure baking treatment (PEB). These treatments can be conductedin substantially the same manner as in the photolithographic patterningworks using a conventional positive-working chemical-amplificationphotoresist composition.

[0025] In the inventive method, the photoresist layer after thepost-exposure baking treatment is subjected to an acid treatment so asto greatly decrease the surface defects in the patterned resist layerformed by a development treatment.

[0026] Namely, it is the inventors' presumption that the defects can bedecreased as a consequence of removal of the alkali-insoluble matter onthe surface after elimination of the acid-dissociablesolubility-reducing groups in the component (A) found on the surface ofthe resist layer by the acid treatment to follow the post-exposurebaking treatment since it is a due assumption that the alkali-insolublematter in the photoresist composition is precipitated in the course ofthe water rinse after the alkali development treatment so as to bedeposited on the surface of the patterned resist layer after thedevelopment treatment forming the defects.

[0027] It is therefore a requirement for the acid solution used in theabove mentioned acid treatment that the acidity of the acid solutionshould be strong enough to cause dissociation of the acid-dissociablesolubility-reducing groups in the component (A) of the photoresistcomposition. In other words, the acid treatment of the photoresist layerafter the post-exposure baking treatment is continued for a sufficientlength of time to cause acid-dissociation of the acid-dissociablesolubility-reducing groups of the compound having hydroxyl groupssubstituted for the hydroxyl-hydrogen atoms by acid-dissociablesolubility-reducing groups found on the surface of the photoresistlayer.

[0028] In consideration of the acid-dissociability of theacid-dissociable solubility-reducing groups of the resinous compound inthe conventional positive-working chemical-amplification photoresistcompositions, the acidic aqueous solution used in the inventive acidtreatment should have a value of pH not exceeding 3.5 or, preferably, inthe range from 1.0 to 3.5. The concentration of the acid in the acidicaqueous solution for the acid treatment is in the range from 0.01 to 50mass % or, preferably, from 0.1 to 10 mass %.

[0029] The acid to prepare the above mentioned acidic aqueous solutioncan be selected from the group consisting of inorganic acids such ashydrochloric and sulfuric acids and organic acids such astrifluoromethane sulfonic acid. In particular, it is sometimesadvantageous that the acid is an organic acid having surface activityincluding fluorinated alkyl sulfonic acids such as perfluoroalkylbutylsulfonic acids, perfluoroalkyloctyl sulfonic acids andperfluoroalkyldecyl sulfonic acids and salts thereof with a quaternaryammonium hydroxide, e.g., tetramethyl-ammonium hydroxide and choline, oran alkanolamine, e.g., ethanolamine and diethanolamine. Among these acidcompounds, non-volatile perfluoroalkyl sulfonic acids are preferred asbeing adaptable to the manufacturing lines of semiconductor devices andalkanolamine salts of a perfluoroalkyl sulfonic acid are preferable inrespect of the easy controllability of the pH value of the aqueoussolution.

[0030] In conducting a screening test for selecting an acid capable ofgiving the most remarkable result of the acid treatment, the inventorshave come to an unexpected discovery that the method of the presentinvention can be accomplished successfully when the acidic aqueoussolution for the acid treatment contains an aromatic sulfonic acid orcarboxylic acid or, in particular, an aromatic sulfonic acid having twobenzene nuclei or an naphthalene ring structure, or a sulfonic acidester having a free acid group as derived from a metal salt of asulfonate or sulfate ester by removing the metal cations.

[0031] Examples of suitable aromatic sulfonic and carboxylic acidsinclude naphthalene carboxylic acids, naphthalene acetic acids,1-naphthol-4-carboxylic acid, 1,8-naphthalene dicarboxylic acid, benzenedisulfonic acids, 1,3,5-benzene trisulfonic acid, phenol sulfonic acid,phenol-2,4-disulfonic acid, cathecol-3,5-disulfonic acid,2-nitrophenol-4-sulfonic acid, biphenyl sulfonic acid, naphthalenesulfonic acid, naphthol sulfonic acid, 2-naphthol-6,8-disulfonic acid,2-naphthol-3,6-disulfonic acid, 4,5-dihydroxy-2,7-naphthalene sulfonicacid, 2,4-dinitro-1-naphthol-7-sulfonic acid.

[0032] Examples of the above mentioned sulfonate acids include alkylbenzene sulfonates having an alkyl group of at least 5 carbon atoms,alkyl naphthalene sulfonates having one or two of isopropyl, butyl orisobutyl groups, polycondensates of a naphthalene sulfonate andformaldehyde, polycondensates of a melamine sulfonate and formaldehyde,disalts of an alkyl sulfosuccinate, dialkyl sulfosuccinates, disalts ofa polyoxyethylene alkyl sulfosuccinate, alkyl sulfoacetates, α-olefinsulfonates, N-acyl-N-methyl-2-aminoethanesulfonates and 5-sodiumsulfodimethyl isophthalate.

[0033] Examples of the above mentioned sulfate ester acids includehigher alcohol sulfate esters, secondary higher alcohol sulfate esters,polyoxyethylene alkylether sulfates, secondary higher alcohol ethoxysulfates and polyoxyethylene alkylphenol ether sulfates.

[0034] A more preferable aromatic acid is a diphenyl ether-basedsulfonic acid represented by the general formula

[0035] in which R¹, R², R³ and R⁴ are, each independently from theothers, a hydrogen atom or an alkyl group having 5 to 20 carbon atomswith the proviso that at least one of the R¹, R², R³ and R⁴ is an alkylgroup having 5 to 20 carbon atoms, and the subscripts p and q are, eachindependently from the other, 0, 1 or 2 with the proviso that p+q is not0. Among the above defined diphenyl ether-based sulfonic acids, mostpreferable in respect of the possibility of decreasing the time for theacid treatment to 10 seconds or shorter are monoalkyl diphenyl etherdisulfonic acids represented by the general formula

[0036] in which R⁵ is an alkyl group having 5 to 20 carbon atoms.

[0037] It is important that the acid compound forming the acidic aqueoussolution has little vaporizability at room temperature not to be lost byvaporization during the acid treatment. For example, such lowvaporizability of the acid compound can be obtained when the acidcompound has a molecular weight of at least 200 or in the range from 200to 1000.

[0038] It is optional that the acidic aqueous solution of the abovementioned acid compound is admixed with a known surface active agenthaving no sulfonic acid group with an object to improve the wettabilityof the photoresist layer with the aqueous solution and to minimize thevolume of the solution ejected from a dispenser nozzle.N-Octyl-2-pyrrolidone is an example of the surface active agent suitablefor the purpose.

[0039] Although, as is mentioned above, the length of time for the acidtreatment according to the invention must be long enough to completelydissociate the solubility-reducing groups of the resinous compoundhaving the acid-dissociable solubility-reducing groups on the surface ofthe photoresist layer, the length of time should be as short as possiblein consideration of the production throughput in the mass production ofsemiconductor devices. From the practical standpoint, the acid treatmentis conducted for 1 to 90 seconds or, preferably, for 1 to 60 seconds. Itis noted that the acid treatment of the photoresist layer after thepost-exposure baking treatment has an effect of decreasing the contactangle of the aqueous alkaline developer solution to the surface of thephotoresist layer.

[0040] The acid treatment according to the inventive method is a processin which the surface of the photoresist layer is brought into contactwith the aqueous acidic solution by spraying the solution at thesurface, by dipping the photoresist layer on the substrate in thesolution or by application of the solution to the surface of thephotoresist layer. In consideration of the requirement for a highproduction throughput, it is preferable in the manufacturing process ofsemiconductor devices that the aqueous acidic solution is applied to thesurface of the photoresist layer, for example, by spin coating underejection of the acid solution from a dispenser nozzle because noparticular additional apparatuses are necessitated therefor.

[0041] The method of the present invention is applicable to thephotoresist layers of positive-working chemical-amplificationphotoresist compositions of any types specifically formulated for thepatterning light exposure by using KrF excimer laser beams, ArF excimerlaser beams, F₂ excimer laser beams, X-rays and electron beams withoutparticular limitations.

[0042] According to the method of the present invention, the acidtreatment of the photoresist layer after the post-exposure bakingtreatment gives a very desirable result of greatly decreasing thesurface defects of the photoresist layer after development. As aconsequence of the trend in the manufacturing technology ofsemiconductor devices toward finer and finer patterning, the fineness ofpatterning of a photoresist layer is required to be 0.15 μm or evenfiner.

[0043] Such a requirement for an increase in the fineness of patterningcan be complied with only with a resinous ingredient having hydroxylgroups substituted by acid-dissociable solubility-reducing groups as abase component in the positive-working chemical-amplificationphotoresist composition in an increased proportion of thehydroxyl-substituting solubility-reducing groups. As the proportion ofthe hydroxyl-substituting solubility-reducing groups is increased,however, the surface defects in the patterned photoresist layer areincreased as a trend so that the increase in the proportion is naturallylimited not to cause a difficulty in obtaining a patterned resist layerof high pattern resolution. By undertaking the method of the presentinvention, this problem of increased surface defects can be solved evenwith a formulation of the photoresist composition comprising a resinousingredient of which the-proportion of the hydroxyl-substitutingsolubility-reducing groups is increased so that a positive-workingchemical-amplification photoresist composition capable of giving a farincreased resolution of the patterned resist layer can be obtained.

[0044] In the following, the method of the present invention isdescribed in more detail by way of Examples and Comparative Exampleswhich, however, never limit the scope of the invention in any way.

EXAMPLE 1

[0045] A positive-working chemical-amplification photoresist solutionwas prepared by dissolving, in 500 parts by weight of propyleneglycolmonomethyl ether acetate, 30 parts by weight of a firstpolyhydroxystyrene resin having a weight-average molecular weight of10000, of which 40% of the hydroxyl-hydrogen atoms were substituted bytetrahydropyranyl groups, 70 parts by weight of a secondpolyhydroxystyrene resin having a weight-average molecular weight of10000, of which 40% of the hydroxylhydrogen atoms were substituted byethoxyethyl groups, 5 parts by weight of bis(cyclohexylsulfonyl)diazomethane, 1 part by weight of bis(4-tert-butylphenyl) iodoniumtrifluoromethane sulfonate, 0.1 part by weight of triethylamine and 0.5part by weight of salicylic acid followed by filtration of the solutionthrough a membrane filter of 0.2 μm pore diameter.

[0046] Separately, two acidic aqueous solutions 1 and 2 were prepared ofwhich the solution 1 was prepared by dissolving 2 g of aperfluoroalkyloctyl sulfonic acid having a molecular weight of 500 in100 g of water and had a pH of 1.7 and solution 2 was prepared byadmixing the solution 1 with 0.051 g of N-octyl-2-pyrrolidone (SurfadoneLP100, a product by ISP Japan Co.) and had a pH of 2.0.

[0047] A 6-inch semiconductor silicon wafer was spin-coated with theabove prepared photoresist solution followed by heating at 90° C. for 90seconds to give a dried photoresist layer having a film thickness of 0.7μm. The photoresist layer was patternwise exposed to KrF excimer laserbeams through a pattern-bearing photomask on a minifying projectionlight-exposure machine (Model FPA-3000EX3, manufactured by Canon Co.)followed by a post-exposure baking treatment at 90° C. for 90 seconds.

[0048] The whole surface of the photoresist layer after thepost-exposure baking treatment was coated on a spinner for 30 secondswith one of the above-prepared acidic aqueous solutions 1 and 2 at roomtemperature ejected out of a dispenser nozzle. Thereafter, thephotoresist layer was subjected to a development treatment with a 2.38mass % aqueous solution of tetramethylammonium hydroxide as a developersolution at 23° C. for 60 seconds followed by rinse with water for 60seconds.

[0049] Each of the thus obtained line-and-space patterned resist layersfor the acidic aqueous solutions 1 and 2 was examined for the crosssectional profile on a scanning electron microscopic photograph to findgood orthogonality. Further, the line-and-space patterned resist layerswere examined for the 1:1 critical pattern resolution, photosensitivity,which was the minimum exposure dose which gave a line-and-space patternof 0.25 μm line width, and the number of the surface defects to give theresults that the critical resolution was 0.20 μm, the photosensitivitywas 35 mJ/cm² and the number of surface defects was 1-5 for each of theacidic solutions 1 and 2. Counting of the surface defects in the resistlayer was undertaken with a surface defect tester (Model KLA,manufactured by KLA Co.).

[0050] For comparison, a control experiment was undertaken in the samemanner as above excepting for the omission of the acid treatment withthe acidic aqueous solution. The results were that the criticalresolution and the photosensitivity were as good as in the cases withthe acid treatment while the number of the surface defects was as manyas 10000.

EXAMPLE 2

[0051] Four acidic aqueous solutions 3 to 6 were prepared, of which thesolution 3 having a pH of 2.0 was a 0.5 mass % aqueous solution of adodecyl(diphenyl ether) disulfonic acid having a molecular weight of 498and expressed by the formula

[0052] solution 4 having a pH of 1.82 was a 0.5 mass % aqueous solutionof trifluoromethane sulfonic acid having a molecular weight of 150,solution 5 having a pH of 1.67 was a 1.0 mass % aqueous solution oftrifluoromethane sulfonic acid and solution 6 having a pH of 1.9 was a5.0 mass % aqueous solution of glycolic acid having a molecular weightof 76.

[0053] An acid treatment of a photoresist layer after the post-exposurebaking treatment was undertaken in substantially the same manner as inExample 1 by using one of the above prepared acidic aqueous solutions 3to 6 except that the length of time for the acid treatment was 10seconds instead of 30 seconds.

[0054] The results obtained in these four experiments were that thecross sectional profile of the patterned resist layer, criticalresolution and photosensitivity were each as good as in Example 1 whilethe number of the surface defects was 0-5 with the solution 3 and 10000with each of the solutions 4 to 6.

What is claimed is:
 1. In a method for the preparation of a patternedphotoresist layer on a substrate surface comprising the steps of (a)forming a layer of a positive-working chemical-amplification photoresistcomposition on the substrate surface, (b) patternwise exposing thephotoresist layer to actinic rays, (c) subjecting the patternwiselight-exposed photoresist layer to a post-exposure baking treatment and(d) subjecting the photoresist layer after the post-exposure bakingtreatment to a development treatment with an aqueous alkaline developersolution, the improvement which comprises bringing the photoresist layerafter the post-exposure baking treatment in step (c), prior to thedevelopment treatment in step (d), into contact with an aqueous acidicsolution containing an acid.
 2. The improvement as claimed in claim 1 inwhich the aqueous acidic solution has a pH of 3.5 or lower.
 3. Theimprovement as claimed in claim 1 in which the concentration of the acidin the aqueous acidic solution is in the range from 0.01 to 50 mass %.4. The improvement as claimed in claim 1 in which the acid contained inthe aqueous acidic solution is an aromatic sulfonic acid.
 5. Theimprovement as claimed in claim 4 in which the aromatic sulfonic acidcontained in the aqueous acidic solution is a compound having twobenzene nuclei or a naphthalene ring structure in a molecule.
 6. Theimprovement as claimed in claim 5 in which the aromatic sulfonic acidcontained in the aqueous acidic solution is a diphenyl ether sulfonicacid represented by the general formula

in which each of R¹, R², R³ and R⁴ is, independently from the others, ahydrogen atom or an alkyl group having 5 to 20 carbon atoms with theproviso that at least one of R¹, R², R³ and R⁴ is an alkyl group having5 to 20 carbon atoms and each of the subscripts p and q is,independently from the other, 0, 1 or 2 with the proviso that p+q is apositive integer.
 7. The improvement as claimed in claim 6 in which thediphenyl ether sulfonic acid is a dodecyl(diphenyl ether) disulfonicacid.
 8. The improvement as claimed in claim 1 in which the photoresistlayer is brought into contact with the aqueous acidic solution byejecting the solution at the photoresist layer.
 9. The improvement asclaimed in claim 1 in which the photoresist layer is kept in contactwith the aqueous acidic solution for a length of time in the range from1 to 90 seconds at room temperature.
 10. The improvement as claimed inclaim 1 in which the aqueous acidic solution contains a surface activeagent.
 11. The improvement as claimed in claim 10 in which the surfaceactive agent is N-octyl-2-pyrrolidone.
 12. The improvement as claimed inclaim 1 in which the acid contained in the aqueous acidic solution has amolecular weight of at least
 200. 13. The improvement as claimed inclaim 1 in which the acid contained in the aqueous acidic solution is anorganic acid having fluorinated alkyl sulfonic acids.